In commercial aircraft, so-called air-supported air conditioning systems are currently conventionally used for air conditioning the aircraft cabin. An aircraft air conditioning system serves for cooling the aircraft cabin, which would otherwise become too hot due to thermal loads such as, for example, solar radiation, body heat from the passengers and waste heat from devices present on board the aircraft. Moreover, the aircraft air conditioning system supplies sufficient fresh air into the aircraft cabin to ensure that a required minimum oxygen content is present in the aircraft cabin.
Air-supported aircraft air conditioning systems generally comprise an air conditioning unit which is supplied with process air compressed by the engine of the aircraft or a separate compressor. Prior to being supplied to the air conditioning unit, the process air is pre-cooled in a preheat exchanger by heat transfer to cooling air which is likewise provided by the engines of the aircraft or a separate compressor. The cooling capacity of the preheat exchanger is controlled by controlling the cooling air mass flow through the preheat exchanger accordingly. In the air conditioning unit, the process air is cooled as it flows through a heat exchanger unit by heat transfer to ambient air flowing through a ram air channel. The ambient air mass flow through the ram air channel, and therefore the cooling capacity of the heat exchanger unit of the air conditioning unit, is controlled by opening and closing corresponding ram air channel flaps.
Cooled process air exiting from the air conditioning unit of an aircraft air conditioning system is then conducted into a mixing chamber and mixed there with recirculation air discharged from an aircraft region to be cooled, for example an aircraft cabin. The mixed air from the mixing chamber is conducted by way of corresponding mixed air lines into the aircraft region to be cooled, which is divided into different air conditioning zones. If necessary, the mixed air lines are supplied downstream of the mixing chamber with hot air, so-called trim air, which is diverted from the process air supplied to the air conditioning unit. The process air supplied to the air conditioning unit, and therefore also the trim air diverted from the process air supplied to the air conditioning unit, conventionally has a constant temperature of ca. 200° C. However, for fault scenarios or certain aircraft-environment conditions, it is also possible to provide a higher or lower process air temperature at the inlet of the air conditioning unit. The temperature of the air in the mixed air lines is therefore controlled by controlling the trim air mass flow supplied to the mixed air lines accordingly.
In general, the operation of an aircraft air conditioning system has an adverse effect on the efficiency of the aircraft in flight mode and the fuel consumption of the aircraft, with the drops in efficiency caused by the operation of the aircraft air conditioning system being greater as the amount of process air which has to be provided by the engines of the aircraft or a separate compressor increases.
Furthermore, an increase in the cooling capacity of the air conditioning unit by opening the ram air channel flaps to increase the ambient air flow through the heat exchanger unit of the air conditioning unit results in an increase in the fuel consumption of the aircraft. These correlations are described in DE 10 2008 053 320 A1.